Grease composition

ABSTRACT

The invention provides a grease composition containing a base oil, a thickener, and an anti-flaking additive such as a compound represented by, for example, formula (1-1), which grease composition can prevent the white layer flaking of the rolling bearings. (In the formula, R1 and R4 are each independently a straight-chain or branched alkyl or alkenyl group having 1 to 20 carbon atoms.)

TECHNICAL FIELD

The present invention relates to a grease composition for preventingwhite layer flaking of the rolling bearings.

BACKGROUND ART

To satisfy the demands on cars for a smaller size and lighter weight onone hand and a larger living space on the other hand, the reduction ofspace for the engine room has been required, which has led to thereduction in size and weight of the parts in the automotive electricalequipment and automotive auxiliaries, such as alternators, tensionpulleys and the like. Also, in response to the demand for quietness, theengine room is closely sealed, so that greases are required to beresistant to high temperatures in consideration of the high-temperatureoperating environments.

In addition, the poly-V belts have been employed since the mid-1980s tomeet the tendencies toward smaller-diameter pulleys and largertransmission torque, and to improve the belt durability. Concurrently, apeculiar problem has been produced, resulting from the flaking thatoccurs at the early stage, associated with white structural change onthe rolling surface of the rolling bearings.

The bearings for use in the automotive electrical equipment orautomotive auxiliaries have been thus required to have both longlubrication life and excellent resistance to flaking.

There are conventionally employed for rolling bearings lithium soapgreases or diurea greases using inexpensive mineral oil as the base oil;lithium soap greases or diurea greases using as the base oil a synthetichydrocarbon oil and an ether type synthetic oil and the like. Inparticular, diurea greases containing the aromatic urea compounds arefrequently chosen in light of the durability under high temperatures.

However, those greases cannot satisfy the long bearing life under hightemperatures because of the insufficient heat resistance of the employedbase oils or thickeners and the poor flowability toward the bearingportions to be lubricated with grease.

In order to inhibit a catalytic action on the metal surface newlyexposed as a result of the wear, an anti-flaking additive, for example,an oxidizer for passivation such as nitrites or the like is added to thegrease composition for oxidizing the metal surface to inhibit thecatalytic action thereof, thereby preventing the generation of hydrogenthat would be caused by decomposition of the lubricant. (JP (Hei)3-210394 A and JP (Hei) 5-263091).

Also, use of a phenyl ether type synthetic oil as the base oil forgrease is proposed to prevent the generation of hydrogen caused bydecomposition of the lubricant (JP (Hei) 3-250094 A).

Further, it is proposed to add an azo compound capable of absorbinghydrogen to the composition of grease to be used for the metal materialsrequired to have tribological properties and for a variety of members,in particular, the grease to be enclosed in the bearing located atportions easily exposed to water (JP 2002-130301 A).

In addition, a grease composition comprising a fluorinated polymer oilas the base oil, polytetrafluoroethylene as the thickener, and anelectroconductive material is proposed for the purpose of extending thelife of rolling bearings, without causing the hydrogenembrittlement-induced flaking even when water permeates through thebearing (JP 2002-250351 A).

Also, there is proposed a grease composition comprising a poly α-olefinsynthetic oil or diphenyl ether type synthetic oil, a urea-basedthickener, at least one of an organic antimony compound or an organicmolybdenum compound as the extreme-pressure agent, and zinc sulfonate(JP 2004-108403 A), which is designed to form a film on the surface ofthe rolling bearing to reduce the load applied to the rolling bearing inthe tangential direction thereof under severe conditions including hightemperatures, high speeds, heavy loads and the like.

However, any of the above-mentioned proposals do not provide sufficientmeasures against the flaking problem after generation of hydrogenbecause those proposals are not intended to cope with the action aftergeneration of hydrogen, in other words, not intended to prevent hydrogenfrom penetrating to the inside of metal.

As the additives for grease, the compounds containing sulfur atom andnitrogen atom, such as thiadiazole compounds are known. For example, JP(Hei) 09-176670A discloses that a thiazole compound such as2,5-dimercapto-1,3,4-thiadiazole and the like can impart awear-resistant action when used in combination with an alkali metalborate.

JP 2002-206095 A discloses that when the grease to be enclosed in therolling bearing for supporting the main shaft of machine tools comprisesa predetermined sulfur-containing compound such as a disulfide compoundor the like, the bearing life can be improved and heat generation can bereduced under the high-speed rotations.

JP 2007-186609 A discloses a grease composition for electric contact.When used within a low temperature region, the grease composition iscapable of effectively reducing wear of the copper surface or copperalloy surface (silver-plated surface, gold-plated surface) withoutcausing any chattering (drop in voltage) even at the contact of a verylow current by adding an organic zinc compound or thiadiazole compoundto the grease composition.

SUMMARY OF INVENTION Technical Problem

An object of the invention is to provide a grease composition capable ofpreventing the white layer flaking of rolling bearings.

Solution to Problem

As a result of intensive studies by the inventors of the invention, itwas found that use of a particular additive can effectively reduce thewhite layer flaking of rolling bearings so that the additive cansuccessfully extend the anti-flaking life of rolling bearings.

Namely, the invention provides a grease composition as shown below.

A grease composition for preventing the white layer flaking of rollingbearings, comprising a base oil, a thickener, and an anti-flakingadditive represented by the following formula (1):

R₁—S_(x)-A  (1)

wherein

R₁ is a hydrogen atom, a straight-chain or branched alkyl or alkenylgroup having 1 to 20 carbon atoms, or an aromatic hydrocarbon grouphaving 6 to 26 carbon atoms;

A is a hydrogen atom, —S_(y)—R₂, —S_(y)—B—R₃, —R₂SH or a grouprepresented by the following formula (2-1) or (2-2):

wherein R₂ is a straight-chain or branched alkyl or alkenyl group having1 to 20 carbon atoms, or an aromatic hydrocarbon group having 6 to 26carbon atoms;

R₅ is a straight-chain or branched alkyl or alkenyl group having 1 to 20carbon atoms, or an aromatic hydrocarbon group having 6 to 26 carbonatoms;

B is a 5-membered heterocyclic ring having at least one heteroatomselected from the group consisting of sulfur atom, nitrogen atom andoxygen atom;

R₃ is a hydrogen atom, —S_(z)—R₄, —SH, a straight-chain or branchedalkyl or alkenyl group having 1 to 20 carbon atoms, or an aromatichydrocarbon group having 6 to 26 carbon atoms;

R₄ is a straight-chain or branched alkyl or alkenyl group having 1 to 20carbon atoms, or an aromatic hydrocarbon group having 6 to 26 carbonatoms;

x is a number from 1 to 10;

y is a number from 0 to 10;

z is a number from 1 to 10; and

w is a number from 1 to 10;

where the straight-chain or branched alkyl or alkenyl group having 1 to20 carbon atoms may optionally comprise one or more heteroatoms selectedfrom the group consisting of nitrogen atom and oxygen atom, and/or maybe optionally substituted by one or more aromatic hydrocarbon groupshaving 6 to 26 carbon atoms; provided that R₁ and A do not represent ahydrogen atom at the same time.

Advantageous Effects of Invention

The grease composition of the invention can effectively prevent thewhite layer flaking of rolling bearings to extend the anti-flaking lifeof the rolling bearings. Also, the grease composition of the inventioncan exhibit a long lubrication life even at an elevated temperature.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic diagram showing the test using four rolling steelballs. In the FIGURE, n indicates 1500 rpm and W indicates 100 kg (4.1GPa).

DESCRIPTION OF EMBODIMENTS

The term “white layer flaking” herein used means abnormal flakingoccurring at the early stage, associated with white color change in thestructure. The term “white layer flaking” herein used is synonymous withthe terms of white flaking, white phase flaking, embrittlement-inducedflaking, hydrogen embrittlement-induced flaking and the like in thisfield.

In general, the bearing life depending on the rolling fatigue can beestimated in accordance with the formula for the life as defined in thestandards (IS00281, JIS B-1518). However, in the case where the whitelayer flaking occurs, the life span may become shorter than estimated.In fact, it is reported that the life span of the rolling bearings onthe market was as short as about 1/10 to 1/20 the estimated life span.The white layer flaking is one of the damages caused byinterior-originating fatigue, and shows a peculiar phenomenon wherewhite layer was exposed when Nital was used to etch the metallicstructure after occurrence of the flaking.

[Base Oil]

The base oil that can be used in the invention is not particularlylimited. Mineral oils and synthetic oils can be used. Those base oilsmay be used alone or in combination.

Examples of the synthetic oils include ester type synthetic oils such asdiesters and polyol esters; hydrocarbon synthetic oils such as polyα-olefins and polybutene; ether type synthetic oils such as alkyldiphenyl ethers, dialkyl diphenyl ethers and polypropylene glycols;silicone oils; fluorinated oils; and other kinds of synthetic oils.

As for the ester type synthetic oils, complex ester oils are preferred,which are synthesized from a polyol (for example, pentaerythritol) witha monovalent fatty acid (for example, a straight-chain or branchedsaturated or unsaturated fatty acid having 6 to 22 carbon atoms, such ascaprylic acid, nonanoic acid or the like) and a polybasic acid (forexample, a straight-chain or branched saturated or unsaturated dibasicacid having 3 to 10 carbon atoms, such as adipic acid or the like). Inparticular, it is preferable to use complex ester oils synthesized frompentaerythritol with adipic acid, heptanoic acid, caprylic acid andcapric acid.

Of the hydrocarbon synthetic oils, poly α-olefins are preferable.

Of the ether type synthetic oils, alkyl diphenyl ethers are preferable.

The synthetic oils are preferably used as the base oils in theinvention. Particularly, the ester type synthetic oils, hydrocarbonsynthetic oils and ether type synthetic oils are preferable. The complexester oils synthesized from pentaerythritol with fatty acids consistingof adipic acid, heptanoic acid, caprylic acid and capric acid; polyα-olefins; and alkyl diphenyl ethers are more preferable.

The base oil may preferably have a kinematic viscosity at 40° C. of 20to 500 mm²/s. When the kinematic viscosity is less than 20 mm²/s at 40°C., a sufficient oil film may not be ensured during the low speedoperation and under high temperatures. When the kinematic viscosityexceeds 500 mm²/s at 40° C., the torque may abnormally increase duringthe high speed operation and under low temperatures. From the samereasons as mentioned above, the kinematic viscosity at 40° C. may bemore preferably 50 to 200 mm²/s, and most preferably 60 to 130 mm²/s.

The content of the base oil may preferably be in the range of 95 to 50mass %, and more preferably 90 to 70 mass %, based on the total mass ofthe grease composition of the invention.

[Thickener]

Under the circumstances of high temperatures, the flowability of thegrease in the bearing varies depending on the kind of thickenercontained in the grease, which has a serious effect on the bearinglubrication life. For obtaining the long lubrication life, the grease isrequired to constantly retain on the portions to be lubricated, withoutsoftening or leakage. In light of this, as the thickener that can beused in the invention, a diurea compound represented by the followingformula (I) is preferable:

R1-NHCONH—R2-NHCONH—R3  (I)

wherein R2 is a bivalent aromatic hydrocarbon group having 6 to 15carbon atoms; and R1 and R3, which may be the same or different fromeach other represent a straight-chain or branched alkyl group having 8to 22 carbon atoms, cyclohexyl group or an aromatic hydrocarbon grouphaving 6 to 12 carbon atoms.

The diurea compound of formula (I) is obtainable by reacting adiisocyanate represented by the following formula (II) with a monoaminerepresented by the following formula (III-1) or (III-2). The reactionconditions for obtaining the diurea compound are well known to personsskilled in the art.

O═C═N—R2-N═C═O  (II)

R1-NH₂  (III-1)

R3-NH₂  (III-2)

(wherein R1, R2 and R3 are the same as those previously defined.)

Representative examples of the group indicated by R² include the groupshaving the following structural formulas. Particularly preferable is thegroup where two phenyl groups are bonded to methylene group, asindicated by (II-2) in the center.

As the alkyl group having 8 to 22 carbon atoms, straight-chain alkylgroups having 8 to 18 carbon atoms are preferred, and straight-chainalkyl groups having 8 carbon atoms and 18 carbon atoms are mostpreferable. As the aromatic hydrocarbon group having 6 to 12 carbonatoms, those having seven carbon atoms are most preferable.

When one of R¹ or R³ represents cyclohexyl group and the otherrepresents a straight-chain or branched alkyl group having 8 to 22carbon atoms in formula (I), the ratio of the number of moles of thecyclohexyl groups to the total number of moles of the cyclohexyl groupsand the straight-chain or branched alkyl groups having 8 to 22 carbonatoms may be in the range of 60 to 95%, and more preferably 70 to 90%.If the above-mentioned molar ratio is less than 60%, the flowabilitytends to increase and the resultant grease tends to soften, so that thegrease easily leaks from the bearing, which will shorten the life. Ifthe above-mentioned molar ratio exceeds 95%, the resultant grease tendsto be so hard that the flowability becomes poor. As a result, the greasecannot easily penetrate into the portion to be lubricated, which mayshorten the life.

Particularly, the diurea compound obtainable by reacting thediisocyanate of formula (II) where R2 is the group represented by theabove formula (II-2) with the monoamine of formula (III-1) where R1 is astraight-chain alkyl group having eight carbon atoms and the monoamineof formula (III-2) where R3 is a straight-chain alkyl group having eightcarbon atoms, that is, the diurea compound represented by the followingformula (I-1) is preferably used.

Also, preferred is a mixture of the diurea compound obtainable byreacting the diisocyanate of formula (II) where R2 is the grouprepresented by the above formula (II-2) with the monoamine of formula(III-1) where R1 is cyclohexyl group and the monoamine of formula(III-2) where R3 is a straight-chain alkyl group having 18 carbon atoms,that is, the diurea compound represented by the following formula(I-2-1); the diurea compound represented by the following formula(I-2-2); and the diurea compound represented by the following formula(I-2-3), with the ratio of the number of moles of the cyclohexyl groupsto the total number of moles of the cyclohexyl groups and thestraight-chain alkyl groups having 18 carbon atoms being in the range of70 to 90 mol %.

Also, the diurea compound obtainable by reacting the diisocyanate offormula (II) where R2 is the group represented by the above formula(II-2) with the monoamine of formula (III-1) where R1 is an aromatichydrocarbon group having seven carbon atoms and the monoamine of formula(III-2) where R3 is an aromatic hydrocarbon group having seven carbonatoms, that is, the diurea compound represented by the following formula(I-3) is particularly preferable.

Most preferable is a mixture of the diurea compounds represented by theabove formulas (I-2-1), (I-2-2) and (I-2-3), with the ratio of thenumber of moles of the cyclohexyl groups to the total number of moles ofthe cyclohexyl groups and the straight-chain alkyl groups having 18carbon atoms being in the range of 70 to 90 mol %.

The content of the thickener may preferably be in the range of 5 to 25mass %, and more preferably 10 to 20 mass %, based on the total mass ofthe grease composition according to the invention. With the content ofless than 5 mass %, the resultant grease will be soft and may cause theproblem of leakage, which cannot satisfy the lubrication life. On theother hand, when the content exceeds 25 mass %, the poor flowabilitywill hinder the grease from entering into the portions to be lubricated,which may make the lubrication life unsatisfactory.

[Worked Penetration]

The worked penetration of the grease composition according to theinvention may preferably be 200 to 300, and more preferably 220 to 280.When the worked penetration exceeds 300, the high-speed revolutions willoften cause the problem of grease leakage, which may hinder thesatisfactory lubrication life. When the worked penetration is less than200, the poor flowability of the resultant grease may not satisfy therequired lubrication life.

[Anti-Flaking Additive]

The anti-flaking additive used in the invention is represented by theabove formula (1). The anti-flaking additive may be used alone or two ormore kinds of anti-flaking additives may be used together.

The anti-flaking additive that is used in the invention includes thefollowing compounds:

-   -   S—H compounds represented by formula (1) where R₁ is a hydrogen        atom;    -   di-, tri-, tetra- or polysulfide bond-containing compounds        represented by formula (1) where R₁ is a straight-chain or        branched alkyl or alkenyl group having 1 to 20 carbon atoms or        an aromatic hydrocarbon group having 6 to 26 carbon atoms; and A        is —S_(y)—R₂ or —S_(y)—B—R₃; and    -   thiazole compounds represented by formula (1) where A is        —S_(y)—B—R₃, in which R₃ represents —S_(z)—R₄, —SH, a        straight-chain or branched alkyl or alkenyl group having 1 to 20        carbon atoms or an aromatic hydrocarbon group having 6 to 26        carbon atoms.

Specific examples of the anti-flaking additive represented by formula(1) include mercaptan, mercaptothiazole and derivatives thereof,mercaptothiadiazole and derivatives thereof, dimercaptothiadiazole andderivatives thereof, sulfurized fats and oils, olefin sulfides,polysulfides, mercaptothiazole benzothiazole, benzothiadiazole and thelike.

More specifically, examples are as follows: diphenyl sulfide,benzylphenyl sulfide, diphenyl disulfide, 4,4′-dimethyldiphenyldisulfide, 2,2′-dipyridyl disulfide, 2,2-dipyrimidine disulfide,bis(benzothiazole-2-yl)persulfide, difurfuryl disulfide, dimethylsulfide, diethyl sulfide, isopropyl sulfide, n-propyl sulfide, isobutylsulfide, di-n-hexyl sulfide, dodecylmethyl sulfide, n-nonyl sulfide,n-dodecyl sulfide, di-n-butyl disulfide, methylpropyl pertrisulfide,bis-(2-mercaptoethyl)sulfide, dibutyl trisulfide, didodecyl trisulfide,dioctyl pentasulfide, didodecyl pentasulfide, dinonyl pentasulfide,dodecyl disulfide, dodecyl trisulfide, dioctyl pentasulfide, dioctyltetrasulfide, o-mercaptobenzoic acid, 2-(methylmercapto)aniline,6-mercaptonicotinic acid, 2-mercaptopyrimidine,(2-mercaptoethyl)pyrazine, 4,6-dimethyl-2-mercaptopyrimidine,4-hydroxy-2-mercapto-6-phenylpyrimidine, 6-mercaptopurine,3-mercapto-1,2,4-azole, 2-dibutylamino-4,6-dimercapto-s-triazine,1-ethyl-5-mercaptotetrazole, 1-methyl-5-mercaptotetrazole,1-(m-acetamidophenyl)-5-mercaptotetrazole,1-(4-ethoxyphenyl)-5-mercaptotetrazole,1-cyclohexyl-5-mercaptotetrazole, 4-methyl-2-mercaptobenzo-thiazole,4,5-dimethylthiazole, 2-isopropyl-4-methylthiazole, 1,2-benzisothiazole,2-methyl-β-naphthothiazole, 2-methoxythiazole, thiazole-2-carbaldehyde,2-acetyl thiazole, benzothiazole-2-acetonitrile, 4-thiazolamine,2-amino-4-methylthiazole, benzothiazol-6-amine, 5-aminobenzothiazole,2-aminobenzothiazole, 2-amino-6-methylbenzothiazole,4-(2-methyl-4-thiazolyl)aniline, 2-amino-5-phenylthiazole,2-amino-6-methoxybenzothiazole, 5-methoxybenzothiazol-2-amine,2-aminobenzo-thiazol-6-ol, 2-amino-α-(methoxyimino)-4-thiazoleaceticacid ethyl ester, 3-isothiazolecarboxylic acid, 4-thiazolecarboxylicacid, 3-methyl-4-isothiazole-carboxylic acid,4-methyl-5-thiazolecarboxylic acid, 2-methylthiazole-5-carboxylic acid,benzothiazole-2-carboxylic acid, 2-benzothiazole acetic acid,4-methyl-5-thiazole ethanol, 2-mercaptobenzothiazole,4-methyl-2-mercaptobenzothiazole, bis(benzothiazol-2-yl)persulfide,2,5-dimercapto-1,3,4-thiadiazole, 2,5-dithioaceticacid-1,3,4-thiadiazole, 2-amino-1,3,4-thiadiazole, 2-thioaceticacid-5-mercapto-1,3,4-thiadiazole, benzenethiol, phenylmethanethiol,p-toluenethiol, 3-methylbenzenethiol, 3,4-dimethylbenzenethiol,2-naphthalenethiol, p-xylenedithiol, toluene-3,4-dithiol,2-aminobenzenethiol, 2-methoxybenzenethiol, 3H-1,2-benzodithiol-3-one,3H-1,2-benzodithiol-3-one 1,1-dioxide, 3-pyridinethiol, 2-pyridinethiol,6-methyl-2-pyridinethiol, 5-nitro-2-pyridinethiol, 1H-imidazole-2-thiol,5-nitro-1H-benzoimidazole-2-thiol, 5-methoxy-1H-benzimidazole-2-thiol,1,1-(thiocarbonyl)-bis(1H-imidazole),6-(dibutylamino)-1,3,5-triazine-2,4-dithiol,1-phenyl-1H-tetrazole-5-thiol, 1,3-dithiol-2-thione,ethylenetrithiocarbonate, 1,3-dithiol-2-thione-4,5-dicarboxylic aciddimethyl ether, 4-methyl-2-mercaptobenzothiazole,5-amino-1,3,4-thiadiazole-2-thiol, bismuthiol,2,2′-dithiobis(1,3,4-thiadiazole-5-thiol), bis[(diethoxyphosphinothioyl)thio]methane,5-heptyl-1,3,4-oxadiazole-2-thiol, methanethiol, 1-propanethiol, n-amylmercaptan, n-hexyl mercaptan, n-hexadecyl mercaptan, stearyl mercaptan,1,3-propanedithiol, 1,4-butanedithiol, 1,5-pentanedithiol,1,6-hexanedithiol, isobutyl mercaptan, 2,3-butanedithiol,1,2-propanedithiol, β-mercaptopropionic acid, triglycol dimercaptan,mercaptopropionic acid methoxybutyl ether,2-ethylhexyl-3-mercaptopropionate, n-octyl-3-mercaptopropionate,mercapto-propionic acid tridecyl ether, stearyl-3-mercaptopropionate,3-mercaptopropionate, bis-(2-mercaptoethyl)sulfide, trimethylol propane,tris(3-mercaptopropionate), dodecyldithiobenzothiazole,hexyldithiobenzothiazole, dodecyldithiobenzimidazole,2,5-bis(dodecyldithio)thiadiazole, 2,5-bis(octyldithio)thiadiazole,2,5-bis(diethyldithio-carbamic acid)thiadiazole, sulfurized fats andoils, olefin sulfides and the like.

In particular, preferably used is a compound of formula (1) wherein R₁is a straight-chain or branched alkyl or alkenyl group having 1 to 20carbon atoms, A is a group represented by —S_(y)—B—R₃, in which B is agroup as shown below:

R₃ represents —S_(z)—R₄, in which R₄ is a straight-chain or branchedalkyl or alkenyl group having 1 to 20 carbon atoms,

x is 2,

y is 0, and

z is 2.

Also, the sulfurized fats and oils, and olefin sulfides are preferable.

Especially, 2,5-dimercapto-1,3,4-thiazole derivatives represented by thefollowing formula (1-1) are preferred.

Of the compounds of formula (1-1), the compound where R1 and R4 are eacha straight-chain or branched alkyl or alkenyl group having 1 to 20carbon atoms is preferable. It is most preferable when R1 and R4 bothrepresent a straight-chain alkyl group having eight carbon atoms.

Without wishing to be bound by any theory, the mechanism which caneffectively prevent the white layer flaking of the rolling bearings byusing the grease composition of the invention comprising theanti-flaking additive represented by formula (1) is considered to be asfollows.

Namely, there are various opinions about the mechanism which causes thewhite layer flaking, and the cause has not yet been identified. However,according to the one opinion, the cause for the white layer flaking ispresumed to be the presence of hydrogen. More specifically, when thegrease is used under a heavy load, the grease will decompose to generatehydrogen. The hydrogen thus generated will penetrate to the inside ofthe steel material of the rolling bearing and then react with carbide atthe grain boundaries. This is considered to result in embrittlement ofthe steel material.

The anti-flaking additive used in the invention has in the molecularstructure thereof at least one sulfur atom, which is readily adsorbed onthe metal surface of the rolling bearing. Through the generation ofiron-mercaptide, a film of iron sulfide is obtained on the surface. Thethus obtained iron sulfide film can prevent hydrogen from penetrating tothe inside of the metal, which is considered to lead to excellentanti-flaking properties.

The content of the anti-flaking additive may preferably be in the rangeof 0.1 to 20 mass %, more preferably 0.5 to 10 mass %, and mostpreferably 0.5 to 4 mass %, based on the total mass of the greasecomposition. With the content of less than 0.1 mass %, a sufficienteffect cannot be expected. When the content exceeds 20 mass %, the costperformance is disadvantageous because the obtainable effect will besaturated.

[Optional Additives]

The grease composition of the invention may further comprise any othergenerally used additives when necessary. For example, the rustinhibitor, load carrying additive, antioxidant and the like may be addedif necessary. The contents of those optional additives may generally be0.5 to 5 mass % based on the total mass of the grease compositionaccording to the invention.

The rust inhibitor includes inorganic and organic ones. Examples of theinorganic rust inhibitor include inorganic metallic salts such as sodiumsilicate, lithium carbonate, potassium carbonate, zinc oxide and thelike. Examples of the organic rust inhibitor include benzoates such assodium benzoate and lithium benzoate, sulfonates such as calciumsulfonate and zinc sulfonate, carboxylates such as zinc naphthenate andsodium sebacate, succinic acid and derivatives thereof such as succinicanhydride and succinic acid half ester, sorbitan esters such as sorbitanmonooleate and sorbitan trioleate, and fatty acid amine salts.

Examples of the load carrying additive include phosphorus-containingcompounds such as phosphate and the like; sulfur-containing compoundssuch as polysulfides, sulfurized fats and oils, and the like;phosphorus-sulfur compounds such as phosphorothionates and the like;thiocarbamates; thiophosphates; and organic phosphates.

The antioxidant is known as an oxidative degradation inhibitor forgrease. The phenol type antioxidants and the amine type antioxidants canbe used.

Examples of the phenol type antioxidants include 2,6-di-t-butyl-p-cresol(BHT), 2,2′-methylenebis(4-methyl-6-t-butylphenol),4,4′-butylidenebis(3-methyl-6-t-butyl-phenol), 2,6-di-t-butylphenol,2,4-dimethyl-6-t-butylphenol, t-butylhydroxy anisole (BHA),4,4′-butylidenebis(3-methyl-6-t-butylphenol),4,4′-methylenebis(2,3-di-t-butylphenol),4,4′-thiobis(3-methyl-6-t-butylphenol),octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate and the like. Inparticular, octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate ispreferably used.

Examples of the amine type antioxidants include N-n-butyl-p-aminophenol,4,4′-tetramethyl-di-aminodiphenylmethane, α-naphthylamine,N-phenyl-α-naphthylamine, phenothiazine, alkyldiphenylamine and thelike. In particular, alkyldiphenylamine is preferably used.

[Bearing]

The grease composition of the invention is used for a variety of rollingbearings in the industrial machines and automobiles. Examples of therolling bearings for the industrial machines include those for variousmotors of the industrial machines, the reduction gears and oil hydrauliccomponents of the industrial robots, the main shaft and reduction gearsof the wind power generators, and the winches for the elevators. Therolling bearings for the automobiles preferably include those for theautomotive electrical equipment or automotive auxiliaries, for example,alternators, electromagnetic clutches for car's air conditioners,intermediate pulleys, idler pulleys, tension pulleys and the like.

EXAMPLES Sample Grease Compositions

Preparation of Sample Grease Compositions

Diphenylmethane diisocyanate was reacted with the predeterminedamount(s) of amine(s) (octylamine, cyclohexylamine, stearylamine,p-toluidine) in each base oil to prepare a base grease. To the basegrease, the base oil and the additives were added to have a workedpenetration of 280 (when determined according to JIS K2220) in a mill,thereby obtaining a grease composition.

The formulation for the sample grease compositions are shown in thefollowing Tables. The components used for the preparation of the samplegrease compositions are as follows.

<Base Oils>

-   -   POE: Complex ester oil synthesized from pentaerythritol with        adipic acid, heptanoic acid, caprylic acid and capric acid        (Kinematic viscosity at 40° C.: 102 mm²/s)    -   PAO: Synthetic hydrocarbon oil (Kinematic viscosity at 40° C.:        68.0 mm²/s)    -   ADE: Alkyldiphenyl ether oil (Kinematic viscosity at 40° C.: 100        mm²/s)    -   MO: Mineral oil (Kinematic viscosity at 40° C.: 90 mm²/s)

The kinematic viscosity of the base oil was determined in accordancewith JIS K 2220 23.

<Thickeners>

-   -   Aliphatic diurea: diurea compound consisting of diphenylmethane        diisocyanate and octylamine.    -   Alicyclic—aliphatic diurea: diurea compound consisting of        diphenylmethane diisocyanate, and a mixture of cyclohexylamine        and stearylamine (with a molar ratio of 5:1).    -   Aromatic diurea: diurea compound consisting of diphenylmethane        diisocyanate and p-toluidine.

<Anti-Flaking Additive>

-   -   Bis(octyldithio)thiazole    -   Olefin sulfide    -   Sulfurized fat and oil    -   NaNO₂ (sodium nitrite)

The term “mass %” in the following Tables means the percentage by massbased on the total mass of each sample grease composition. All thesample grease compositions contained the following antioxidant and rustinhibitor although they are not indicated in the Tables.

<Test Method>

-   -   Test using four rolling steel balls

(Outline of the Test)

As shown in FIG. 1, three steel balls with a diameter of 15 mm designedfor bearings were disposed in a cylindrical container with an innerdiameter of 40 mm and a height of 14 mm, which was filled with about 20g of each test grease composition. Another steel ball (⅝-in) for bearingwas set to a tester so that the steel ball (⅝-in) was placed in contactwith the top of the three steel balls. The steel ball (⅝-in) was drivento rotate for 4 hours for shakedown with the application of a load in adirection of W as shown in FIG. 1, and then hydrogen gas was introducedinto the tester. The lower three balls revolved as each rotating on itsaxis. The ball was driven to rotate continuously until the flaking tookplace on the steel ball surfaces.

-   -   The flaking occurs at a point between two balls where the        highest contact pressure is applied.    -   The life was expressed as the total number of contact times of        the upper ball with the lower balls counted when the flaking        took place. These procedures were repeated at least five times        to determine the L50 life (i.e., the number of contact times        when 50% expired). The results are shown in Tables 1 and 2.

(Test Conditions)

-   -   Steel balls for test: 15-mm-dia. steel balls and a ⅝-in steel        ball for bearing    -   Load for test (W): 100 kgf (4.1 GPa)    -   Rotational speed (n): 1500 rpm    -   Feed rate of hydrogen gas: 15 ml/min.    -   Atmospheric pressure at test part: 0.96 atm. (because of vacuum        extraction)    -   The repeated number of tests: 5 (at a minimum)

(Evaluations)

-   -   20×10⁶ times or more: o (acceptable)    -   Less than 20×10⁶ times: x (unacceptable)

TABLE 1 Examples 1 2 3 4 5 6 7 8 Base oils ADE Balance — — — BalanceBalance Balance Balance POE — Balance — — — — — — PAO — — Balance — — —— — MO — — — Balance — — — — Thickeners Aliphatic diurea 10  10  10  10 — — — — (mass %) Alicyclic - — — — — 10  — 10  10  aliphatic diureaAromatic diurea — — — — — 19  — — Anti-flaking Bis(octyldithio)- 2 2 2 22 2 — — additives thiazole (mass %) Olefin sulfide — — — — — — 2 —Sulfurized oil fat — — — — — — — 2 Test using Number of 20< 20< 20< 20<20< 20< 20< 20< four rolling contacts L50 steel balls (×10⁶) Evaluations∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘

TABLE 2 Comparative Examples 1 2 3 4 5 6 7 Base oils ADE Balance Balance— — — Balance Balance POE — — Balance — — — — PAO — — — Balance — — — MO— — — — Balance — — Thickeners Aliphatic diurea 10 10 10 10 10 — — (mass%) Alicyclic - — — — — — 10 — aliphatic diurea Aromatic diurea — — — — —— 19 Anti-flaking NaNO₂ — 2 2 2 2 2 2 additive (mass %) Test usingNumber of 1.2 12.8 8.7 9.5 10.2 14.2 15.5 four rolling contacts L50steel balls (×10⁶) Evaluations x x x x x x x

1. A grease composition for preventing white layer flaking of rollingbearings, comprising a base oil, a thickener, and an anti-flakingadditive represented by formula (1):R₁—S_(x)-A  (1) wherein R₁ is a hydrogen atom, a straight-chain orbranched alkyl or alkenyl group having 1 to 20 carbon atoms, or anaromatic hydrocarbon group having 6 to 26 carbon atoms; A is a hydrogenatom, —S_(y)—R₂, —S_(y)—B—R₃, —R₂SH or a group represented by formula(2-1) or (2-2):

wherein R₂ is a straight-chain or branched alkyl or alkenyl group having1 to 20 carbon atoms, or an aromatic hydrocarbon group having 6 to 26carbon atoms; R₅ is a straight-chain or branched alkyl or alkenyl grouphaving 1 to 20 carbon atoms, or an aromatic hydrocarbon group having 6to 26 carbon atoms; B is a 5-membered heterocyclic ring having at leastone heteroatom selected from the group consisting of sulfur atom,nitrogen atom and oxygen atom; R₃ is a hydrogen atom, —S_(z)—R₄, —SH, astraight-chain or branched alkyl or alkenyl group having 1 to 20 carbonatoms, or an aromatic hydrocarbon group having 6 to 26 carbon atoms; R₄is a straight-chain or branched alkyl or alkenyl group having 1 to 20carbon atoms, or an aromatic hydrocarbon group having 6 to 26 carbonatoms; x is a number from 1 to 10; y is a number from 0 to 10; z is anumber from 1 to 10; and w is a number from 1 to 10; where thestraight-chain or branched alkyl or alkenyl group having 1 to 20 carbonatoms may optionally comprise one or more heteroatoms selected from thegroup consisting of nitrogen atom and oxygen atom, and/or may beoptionally substituted by one or more aromatic hydrocarbon groups having6 to 26 carbon atoms; provided that R₁ and A do not represent a hydrogenatom at the same time.
 2. The grease composition of claim 1, wherein theanti-flaking additive is a compound of formula (1) wherein R₁ is astraight-chain or branched alkyl or alkenyl group having 1 to 20 carbonatoms, A is the group represented by —S_(y)—B—R₃, in which B is a grouprepresented by the following formula:

R₃ represents —S_(z)—R₄, in which R₄ is a straight-chain or branchedalkyl or alkenyl group having 1 to 20 carbon atoms, x is 2, y is 0, andz is 2; or the anti-flaking additive is a sulfurized oil fat, or anolefin sulfide.
 3. The grease composition of claim 1, wherein theanti-flaking additive is: a 2,5-dimercapto-1,3,4-thiazole derivative offormula (1-1):

wherein R1 and R4 are each a straight-chain or branched alkyl or alkenylgroup having 1 to 20 carbon atoms; the sulfurized oil fat; or the olefinsulfide.
 4. The grease composition of claim 3, wherein the anti-flakingadditive is the 2,5-dimercapto-1,3,4-thiazole derivative of formula(1-1) wherein R1 and R4 are both straight-chain alkyl groups havingeight carbon atoms; the sulfurized oil fat; or the olefin sulfide. 5.The grease composition of claim 1, wherein the anti-flaking additive iscontained in an amount of 0.1 to 20 mass % based on the total mass ofthe grease composition.
 6. The grease composition of claim 1, whereinthe thickener is a diurea compound represented by formula (I):R1-NHCONH—R2-NHCONH—R3  (I) wherein R² is a bivalent aromatichydrocarbon group having 6 to 15 carbon atoms; and R¹ and R³ which maybe the same or different from each other represent a straight-chain orbranched alkyl group having 8 to 22 carbon atoms, cyclohexyl group, oran aromatic hydrocarbon group having 6 to 12 carbon atoms.
 7. The greasecomposition of claim 6, wherein the thickener is a diurea compoundrepresented by formula (I-1):

a mixture of a diurea compound represented by formula (I-2-1), a diureacompound represented by formula (I-2-2) and a diurea compoundrepresented by formula (I-2-3),

with the ratio of the number of moles of the cyclohexyl groups to thetotal number of moles of the cyclohexyl groups and the straight-chainalkyl groups having 18 carbon atoms being in the range of 70 to 90 mol%; or a diurea compound represented by formula (I-3).


8. The grease composition of claim 1, wherein the base oil is a mineraloil, a synthetic oil or a mixture thereof.
 9. The grease composition ofclaim 8, wherein the base oil is the synthetic oil selected from thegroup consisting of ester type synthetic oils, hydrocarbon syntheticoils, and ether type synthetic oils.
 10. The grease composition of claim1, wherein the base oil has a kinematic viscosity at 40° C. of 20 to 500mm²/s.